Star vs Delta - advantages & disadvantages

STAR (WYE) ADVANTAGES
- Y used for high voltage
- common neutral point
- neutral point can be earthed
- availability of single phase at a lower voltage
- ability to spread load between the phases
- good for unbalanced loading
- each phase is a separate circuit
- dual voltage applications (3-phase and 4-wire system)
- star connected alternator requires lesser number of turns than delta for the same voltage
- star connected alternator requires lesser insulation
- star connection can distribute the load evenly



STAR (Y) DISADVANTAGES
- less torque
- construction involves combining 3 single phases into 1
- secondary distribution, light duty applications
- construction cost is more expensive (4 conductors)



DELTA ADVANTAGES
- efficient
- more torque
- simple motor design
- protection is simple and less costly
- construction cost is cheaper (3 conductors instead of 4)
- heavy duty applications
- used in rotatory conveyors
- less current per winding for the same power output
- major applications in power generation, transmission and distribution
- the transformer secondary provides all the 3 phases (if one is lost, you still have 2)



DELTA DISADVANTAGES
- no common neutral point
- detecting earth ground faults is difficult
- low voltage connection




Note 1:
If Power is kept constant,
Delta (parallel) has lower voltage but higher current (amperage) capacity
Star (series) has higher voltage but lower amperes

Note 2:
Some motors are soft started with star and then switched to delta.

Note 3:
Most modern motors have windings connected in delta.
Most 3-phase induction motors are connected in delta.

Fractional HP Approx Fractional KW

1 1/3 HP --- 1 KW

1 HP --- 3/4 KW or 750 W

3/4 HP --- 1/2 KW or 500 W

1/2 HP --- 1/3 KW or 375 W

1/4 HP --- 1/5 KW or 200 W

1/5 HP --- 3/20 KW or 150 W

1/6 HP --- 3/25 KW or 120 W

1/8 HP --- 1/10 KW or 100 W

1/10 HP --- 7/100 KW or 70 W

1/12 HP --- 3/50 KW or 60 W

1/15 HP --- 1/20 KW or 50 W

1/20 HP --- 1/25 KW or 40 W

1/25 HP --- 3/100 KW or 30 W

1/50 HP --- 15/1000 KW or 15 W

1/125 HP --- 6/1000 KW or 6 W

HMCS PROTECTEUR 130 TON CHILLER

Chiller Units: 2 Chillers, 130 ton each
Compressor: 150 HP motor per chiller

HVAC: 4 AC units
Fan coil units: 46 fan coil units

Complement: 365 crew

Displacement: 24,700 tons full load
Length:     171.9 m (564 ft 0 in)
Beam:     23.2 m (76 ft 1 in)
Draught:     10.1 m (33 ft 2 in)

Propulsion: 21,000 shp (16,000 kW) steam turbine
Speed:     20 knots (37 km/h; 23 mph)
Range:     7,500 nmi (13,900 km; 8,600 mi) at 11.5 knots

COMPARISON OF REFRIGERATION & HVAC COMPRESSORS

VOLUMETRIC EFFICIENCIES

Reciprocating compressor:  65% to 92%

Rotary compressor:  87% to 92%

Scroll compressor:  > 95%

Screw compressor:  87% to 94%

Centrifugal compressor:  > 99%



CAPACITY (TONNAGE) USAGE

Reciprocating compressor: below 5 tons, below 10 tons, 10-20 tons, 5-150 tons   

Rotary compressor: below 5 tons

Scroll compressor:  5 to 10 tons

Screw compressor:  20 to 750 tons 

Centrifugal compressor: > 150 tons, 10,000-20,000 tons

 

INSTALLATIONS/APPLICATIONS

Reciprocating compressor:
refrigerators, freezers, ice machines, residential air conditioning, heat pumps, split air conditioners, residential to light commercial

Rotary compressor:
home appliances, refrigerators, room air-conditioners, window units, residential central air conditioning, heat pumps

Scroll compressor:
automobile air conditioning systems, heat pumps, freezers, water coolers, large residential, light commercial Central systems, commercial chillers

Screw compressor:
commercial chillers, large commercial and industrial market  

Centrifugal compressor:
industrial and commercial cooling and air conditioning, heat pump, commercial and industrial chillers, largest systems



ADVANTAGES:

Reciprocating compressor:
most commonly used, versatile, robust and powerful, durable (endurance), economical to operate, low initial cost, easy to install, reaches high pressures  

Rotary compressor:
high reliability, high durability, quiet, very efficient, few moving parts, low rotational speeds, low initial cost, low maintenance costs, usable in dirty environments

Scroll compressor:
reliable, very durable, very efficient, lesser moving parts, smoother and quieter operation, less torque variation, 

Screw compressor: 
can produce high pressure, consume less power than reciprocating compressors, low to medium initial and maintenance costs, few moving parts

Centrifugal compressor: 
pumping efficiency increases with speed, no valves, pistons or cylinders, few moving parts


DISADVANTAGES:

Reciprocating compressor:
vibrational issues, noisy, requires maintenance

Rotary compressor:
used on small systems, limited to smaller volumes, lesser pressure developed

Scroll compressor:   
typically more expensive, cooling is difficult compared to piston compressors

Screw compressor: 
difficulty in dirty environments, high rotational speeds, shorter life expectancies

Centrifugal compressor: 
operate at high speeds (e.g. 25,000 rpm), main bearings commonly subject to wear and tear, not positive displacement (fixed volume chamber/casing) 

Induction vs Synchronous Motors: Advantages & Disadvantages comparison

The purpose of this article is to simplify the comparison, differences, similarities, advantages (benefits) and disadvantages of AC Synchronous and AC Induction Motors.

70% of all the energy produced in the world are utilized by electric motors. 70 % of all industrial machines in the world account for 3-phase AC Induction Motors. The vast majority of motors used globally are induction motors. These motors are used everywhere in our daily lives in homes and offices.

Thus, it is essential to know the basic information about these motors in order to make an informed and intelligent decision when buying.



AC SYNCHRONOUS MOTOR Advantages: 

Efficiency:
- higher efficiency than induction motors (Permanent Magnet Synchronous Motor efficiencies are between 93% and 98%)
- more efficient than induction machines because there are no conductor losses in the rotor  
- synchronous reluctance motors have no rotor losses

Power Factor:
- flexible power factor (lagging, unity or leading)
- can be used to improve the power factor of the system
- used to make the overall system power factor higher (e.g. utility companies charge a power factor penalty if your power factor falls below 0.90 )
- can operate from lagging power factor to leading power factor by simply changing its excitation
- possible to alter the power factor by varying the excitation of the rotating DC field supplied to the motor
- synchronous motors can supply reactive power to counteract lagging power factor caused by inductive loads
- when DC field excitation is increased, the power factor (measured at the motor terminals) becomes more leading
- when DC field excitation is decreased, the power factor of the motor becomes more lagging

Power:
- synchronous motors are best suited for very high power at lower speeds
- power varies linearly with the applied voltage
- provide higher power density (ratio of output power to physical size or volume of motor) because of higher magnetic flux compared to induction motor
- no external power supply for mechanical braking needed

Torque:
- switched-reluctance (SR) synchronous motors produces 100% torque at “stall” indefinitely

Speed:
- speed is constant at any load
- speed is not dependent on the load
- high speed operation helps to eliminate power transmission elements such as gearboxes
- more accurate, more precise speed control (even though voltage, temperature, and load fluctuate constantly)
- SR synchronous motors can operate at higher speeds

Current:
- inrush currents are low

Life:
- Permanent Magnet Synchronous motors operate cooler, which means longer bearing life and longer insulation life
- wider air gaps (less vibration, more stable)


*** SynRM: The Motor of the Future
Synchronous Reluctance Motor (SynRM) are predicted to be the future of motor technology and will be the global dominant motor especially in the
industrial facilities as well as in electric vehicles (electric cars, e-bikes, traction electric vehicles, etc.). The following are its main advantages:
- more economical than an induction motor of equivalent frame size (lesser motor size but provides equivalent power output)
- highly efficient
- 6% energy cost savings compared to traditional induction motor
- 75% reduction in audible noice (quiet operation)
- 58% reduction in frame temperatures (cooler operation)
- no permanent magnets (rotor is made of ferro-magnetic material; no need for rare-earth magnets)
- no windings in the rotor
- no rotor losses
- cooler bearing temperature, longer bearing life (bearing failure accounts for 70 percent of all motor failures)

Syn RMs with variable speed drive (VSD) are cheaper, more efficient, smaller, lower maintenance, lower operational cost and with longer lifetime than any type of motor.



AC SYNCHRONOUS MOTOR Disadvantages:

Efficiency:
- permanent magnet synchronous motors are subject to demagnetization (loss of magnetic properties) when operated at high current or high temperatures

Power:
- synchronous motor is doubly excited (stator is supplied with AC power & rotor is supplied with a DC source)
- needs DC excitation from external sources

Torque:
- starting torque is zero

Speed:
- to achieve variable speeds, it needs Variable Frequency Drives to adjust supply frequency

Starting:
- not self starting (needs starting devices)
- cannot be started with a load applied

Control:
- commercially available permanent magnet motors need a variable frequency drive (VFD) to function

Cost:
- more expensive than induction motor with same power rating
- more complicated and more expensive to build

Loading, Stability:
- hunting (when sudden or variable loads are applied, the motor hunts (swings/seeks equilibrium) 
- when overloaded, the motor stops

Safety:
- synchronous motors with collector rings and brushes produce sparks/arcs, ozone gas (O3), wear due to friction and require maintenance



AC INDUCTION MOTOR Advantages:

Efficiency:
- used when huge power at the highest efficiency is required

Power:
- an induction motor is singly excited (AC power is supplied to the stator only)

Speed:
- induction motors are best suited for high speed

Torque:
- 3-phase induction motors (IMs) have high starting torques (maximum slip & maximum torque at zero speed)

Starting:
- three-phase induction motors are self-starting

Maintenance:
- minimal or no maintenance
- lesser parts to maintain
- no brushes, commutators, slip rings

Construction:
- rugged, sturdy and strong construction
- simple, easy to manufacture
- flexible, robust, operates in any environmental condition

Safety:
- no noises (electromagnetic interference, EMI)
- safer in explosive applications (no arcing, no sparking, no fire hazards)

Operation:
- rotating magnetic field is very smooth
- can operate in most severe environments (harsh environmental conditions, extreme temperatures, etc.)

Availability:
- available in a wide range of sizes from a fraction of a horsepower to thousands of horsepower

Cost:
- cheaper cost

Life:
- longer operational life (average 30 years) compared to synchronous motor that has brushes
.


AC INDUCTION MOTOR Disadvantages:

Efficiency:
- high I^2R losses
- lower efficiency compared to synchronous motors
- single-phase induction motors are less efficient than 3-phase induction motors
- IMs with a short circuit rotor cage (squirrel-cage) have rotor losses of 20–35 percent of the total motor losses

Power:
- 3-phase induction motors require 3 power supply lines (not usually available in residential homes)
- single-phase motors are limited to power ratings up to 15 HP
- larger power are available at three-phase (if you only have one-phase power supply, you need a phase converter device)

Power Factor:
- power factor is always lagging (low power factors at light loads)
- power factor of 0.85 to 0.90 at full load
- power factor of 0.20 to 0.40 at no-load
- power factor correction (e.g. static capacitors) are needed

Speed:
- motor speed is always lesser than the synchronous speed
- do not rotate at the same frequency as the alternating drive current
- turns slightly slower than the AC current frequency because of slip
- speed is dependent on the load
- speed decreases when the load increases
- because their speed is hard to control, variable frequency drives are necessary for effective speed control

Torque:
- cannot produce torque without slip (the difference in the stator’s magnetic speed and the rotor speed; rotor "catches up” with the magnetic field)

Current:
- huge starting current (inrush current) 8-10 times the rated full load current (this causes line voltage drop)

Starting:
- single phase induction motors are not self-starting (requires starting device, e.g. capacitor)





Glossary:
 Rotor = rotating part of an alternator, generator, motor, dynamo

 Stator = stationary part of an alternator, generator, motor, dynamo

 Armature = power-producing component of an alternator, generator, motor, dynamo (can be located on the rotor or stator)

 Field = magnetic field component of an alternator, generator, motor, dynamo (can be located on the rotor or stator; can be an electromagnet or a permanent magnet)

 Reluctance = is magnetic reluctance, or magnetic resistance property in which magnetic energy is stored (rather than dissipated as heat, etc); a magnetic field causes magnetic flux to follow the path of least magnetic reluctance.

What is bearing wipe, scoring?

Bearing wipe and scoring:
- most common bearing failures due to insufficient or contaminated lubrication

Wiping of bearing: 
- white metal (babbitt) from the bearing surface melts and then hardens in the cooler area of the bearing
- wiping is caused by loss of hydrodynamic oil film, insufficient oil supply 
- wiped bearing is also caused by lesser clearance due to overheating (thermal expansion)

Troubleshooting/Repair of bearing wipe:
- ensure the rotor is properly balanced
- ensure coupling is correctly aligned
- ensure clean and sufficient supply of lubricating oil
- clean all the dirt
- replace the wiped bearing with a new one, increase the clearance


Scoring of bearing:
- scoring in a bearing is the result of metal-to-metal contact
- evidenced by circular scratches or grooves around the circumference of the bearing surface

Causes of Scored Bearing:
- contamination in the lubricating oil
- dirt, foreign particles, wear particles from metal 

Troubleshooting/Repair of scored bearing:
- clean the journal, oil passages, oil filter
- ensure clean supply of lubricating oil
- replace with new journal bearing

Temperature at which steel fails and babbitt melts

STEEL:
Steel is an alloy of iron and carbon.

Temperature Failure of Steel:
400 C (750 F) - structural steel starts losing yield strength
600 C (1112 F) - structural steel is 50% failed, steel expands and twists
800 C (1470 F) - ordinary construction steel is 90% failed
1000 C (1830 F) - carbon steel is 97% failed
1100 C (2012 F) - carbon steel is 100% failed, complete failure


Steel starts to become soft:
538 °C  (1,000 °F)


Melting Point of Steel:
Carbon Steel:     1425 - 1540 Celsius
Carbon Steel:     2600 - 2800 Fahrenheit
Average Steel:     1400-1600 C depending on type of steel 
Steel:         1370 degrees C  (2500 °F)



   
BABBITT:
Babbitt is a metal alloy for bearing surfaces.

Babbitt is an alloy of  89% Tin,  8% Antimony and  3% Copper

Babbit fatigue strength failure temperature is:  130 C  (265 ˚F)

Babbit melting temperature is:  238 C  ( 460 ˚F )